Effects of radiation on fine vasculature may determine the response of dependent tissues after a course of radiotherapy. Additional stress to irradiated tissues may cause injury to fine vasculature leading to degeneration and necrosis. It is difficult to study radiation dose response and repair capability of nonproliferating or slowly proliferating mammalian tissues. However, knowledge about radiation repair by normal tissues becomes critical when considering use of radiotherapy techniques which may alter this capability. The primary objective of this research is to determine single dose and split-dose radiation survival curves for endothelial cells. Injury to the cornea of the eye will cause proliferation of capillary endothelial cells at the limbus. The dose required to prevent neovascularization of the cornea in 50% of eyes of dogs (NVD50) is about 1000 rads as determined from previous work. Split-dose studies have shown the repair capability for endothelial cells to be 350 rads. Irradiation prior to the following induction of capillary proliferation permits evaluation of response of both slowly and rapidly proliferating capillary endothelial cells. Experiments will be done with both low LET and high LET radiation. Of particular interest is the study of combined effects of ionizing and thermal irradiation, as this may ultimately have practical radiotherapeutic application. Studies will also be done with combined ionizing radiation and chemotherapeutic agents. BIBLIOGRAPHIC REFERENCES: Gillette, E. L., G. D. Mauer, D. J. Clow and G. A. Severin: 1975. Radiation response of endothelial cells. Radiation Research, 62: 610. Abstract of paper presented at the 23rd Annual Meeting of the Radiation Research Society. Miami Beach, Florida, 1975. Fike, J. R., E. L. Gillette and D. J. Clow: 1976. Response of endothelial cells to neutrons and x radiation. In Press. Radiation Research Abstract. Paper to be presented at the 24th Annual Meeting of the Radiation Research Society. San Francisco, 1976.